1. Field of the Invention
The present invention concerns a radio frequency (RF) based system for ablating tissue and occlusions, particularly within liquid-filled lumens of animals, such as the heart, liver, arteries and vessels of a human, with an electrical field produced about an RF antenna, and is particularly concerned with reducing reflected signal losses in such a system.
2. Related Art
Therapeutic tissue ablation systems apply energy to a biological ablation tissue site via different energy exchange means, such as heat conduction and irradiation. These systems may employ various energy modes, such as radiofrequency, ultrasound, laser, cryogenic, and the like. Within the radio frequency (RF) range, certain microwave ablation systems are used to destroy or ablate biological tissues. In one application, a microwave ablation system is used to ablate cardiac tissues that cause irregular heartbeats or arrhythmia, avoiding the need for more risky and invasive open heart surgery. In such an application, an ablation member such as an RF antenna is incorporated as part of a catheter or probe. The catheter is passed through the vein for access to the atrium. Within the atrium, the RF antenna is positioned at the desired location where ablation is applied.
Microwave ablation systems can also be used in treatment of other biological sites such as arteries, organs and body vessels. As an example, a microwave ablation system is used to ablate tumors in the lungs, liver, kidney or other areas of the body.
These surgical and therapeutic applications require an efficient system for the transmission of radio frequency energy to the ablating member for the delivery of energy to the target tissue site. U.S. Patent Application Publication No. 20080015570 of Ormsby et al. describes a tissue ablation system comprising a hollow conductive coaxial cable having a first inner elongated electrically conductive tubular member having a distal end portion, the first tubular member having a hollow, axially extending lumen, a second elongated electrically conductive member disposed in a substantially coaxial relationship over at least a portion of the first electrically conductive tubular member over substantially the length of the cable, a dielectric medium disposed between the first and second electrically conductive tubular members, and an ablating member or radio-frequency antenna which delivers radio frequency energy including microwaves to body tissue disposed at the distal end portion of the cable. The radio-frequency antenna is adapted to receive and irradiate radio-frequency energy in the microwave range at a frequency typically greater than 300 Megahertz (MHz) in the electromagnetic spectrum for ablating biological tissue along a biological ablation pathway.
Typical microwave tissue ablation systems have a RF power supply which provides RF energy along the coaxial cable or waveguide to the antenna. Most current tissue ablation systems are designed to provide a set impedance, which may be of the order of 50 ohms. However, the impedance on the catheter side of the system tends to vary, for example due to coaxial cable characteristics and variations in the electromagnetic properties of the tissue under treatment. It is known that the dielectric constants of different types of tissue, for example heart tissue and liver tissue, are different. Also, the dielectric properties of the tissue change as the tissue is treated. This prevents a fixed electromagnetic RF supply circuit from achieving maximum performance in delivering the highest amount of RF energy to the tissue being treated. In U.S. Pat. No. 6,190,382 of Ormsby, a microstrip transformer between the RF energy source and the transmission line or catheter is used to adapt the 50 ohm system more closely to the ablation antenna impedance.
In U.S. Pat. No. 7,070,595 of Ormsby et al., a tissue ablation system and method is described in which the output frequency of the RF energy pulses supplied to the catheter is adjusted to effect a substantial match with the RF antenna and biological tissue load impedence. In this system, a bi-directional coupler samples the forward pulses supplied to the microwave transmission line or co-axial cable and the reflected pulses which are reflected from the target ablation tissue, and uses the signal samples as feedback to a controller which varies the frequency in order to reduce the reflected signal, so that more energy is applied to the tissue undergoing ablation. U.S. Pat. No. 5,957,969 of Warner et al. describes a mechanically tuned microwave ablation catheter system and method which has a tuner located in the power supply, the transmission line, or the antenna which changes the antenna configuration, moves material relative to the antenna, or alters the waveguide.